A liquid-crystal display (LCD) device comprises a liquid-crystal cell and a polarizing plate. The polarizing plate has a polarizing film and a protective film, and is fabricated, for example, by coloring a polarizing film of polyvinyl alcohol with iodine, stretching it and attaching a protective film to both surfaces of the film. In a transmission-type liquid-crystal display device, such a polarizing plate is disposed on both sides of the liquid-crystal cell therein, and one or more optically-compensatory films may be further disposed therein. In a reflection-type liquid-crystal display device, a reflector, a liquid-crystal cell, one or more optically-compensatory films and a polarizing plate are disposed in that order. The protective film or the optically-compensatory film disposed between two polarizing films or between a polarizing film and a reflector have significant influences on the display capabilities of the liquid-crystal display devices, and their functions are extremely important.
A liquid-crystal cell comprises liquid-crystalline molecules, two substrates for sealing up them therebetween, and an electrode layer for imparting a voltage to the liquid-crystalline molecules. The liquid-crystal cell acts for ON/OFF display, depending on the difference in the alignment condition of the liquid-crystalline molecules therein, and for it, proposed are various display modes of TN (twisted nematic), IPS (in-plane switching), OCB (optically-compensatory bent), VA (vertically-aligned), ECB (electrically-controlled birefringence) modes that are applicable to any of transmission-type and reflection-type liquid-crystal display devices.
Of such LCDs, a TN-mode liquid-crystal display device (90-degree twisted nematic-mode liquid-crystal display device) is suitable to applications that require high-level display quality. The TN-mode device comprises nematic liquid-crystal molecules having positive dielectric anisotropy, and is driven by a thin-film transistor. The TN-mode device has a viewing angle characteristic in that, when seen in front of it, the device shows excellent display characteristics, but when seen in the oblique direction thereof, its contrast lowers and its brightness in gradation display is reversed for gradation reversal and its display characteristic is thereby worsened, and it is strongly desired to improve this characteristic.
For solving the problem, proposed are an IPS (in-plane switching) mode liquid-crystal display where a lateral electric field is applied to the liquid crystal therein, and a VA (vertically-aligned) mode device where the liquid-crystal molecules having negative dielectric anisotropy are vertically aligned for alignment division by the projections and the slit electrodes formed inside the panel, and these are now in practical use. Recently, these panels have been developed not only for monitors for personal computers and the like but also for TVs, and accordingly, the plane brightness of the panels has become significantly increased. As a result, in these devices, a minor light leakage in the oblique incident direction in the diagonal position in black display, which has heretofore been negligible in these driving modes, has become problematic as a cause of worsening the display quality.
As one method for improving the color tone and the viewing angle characteristic in black display, disposing an optically-compensatory material having a birefringence characteristic between a liquid-crystal layer and a polarizing plate in an IPS-mode device is investigated. For example, disclosed is a technique of improving the color appearance in direct viewing of white display or intermediate tone display in the oblique direction of a device by disposing a birefringence medium, in which the optical axes having an action of compensating the change in the retardation of the liquid-crystal layer in inclination are put perpendicularly to each other, between a substrate and a polarizing plate (see JP-A 9-80424). Also proposed is a method of using an optically-compensatory film that comprises a styrenic polymer having a negative intrinsic birefringence and a discotic liquid-crystalline compound (see JP-A 10-54982, JP-A 11-202323 and JP-A 9-292522).
Many of the proposed methods as above are for improving the viewing angle characteristic by canceling the birefringence anisotropy of the liquid crystal in the liquid-crystal cell, and are therefore still problematic in that the problem of light leakage caused by the crossing angle shifting of the polarizing axes from perpendicular crossing when the orthogonal polarizing plate is seen in the oblique direction could not be satisfactorily solved. Even in the system that may be able to compensate the light leakage, it is extremely difficult to completely optically compensate the liquid-crystal cell with no problem. This is because, even though the light leakage could be completely compensated at a certain wavelength, it is not always possible to compensate it at any other wavelength. For example, even when light passage at a wavelength of green that has a largest visual sensitivity could be compensated, there still remains a problem in that light leakage at a shorter wavelength of blue or at a longer wavelength of red may occur. For solving the problem, Jpn. J. Appl. Phys., 41, (2002), 4553, has proposed a technique of layering two biaxial films.
However, since the method of Jpn. J. Appl. Phys., 41, (2002), 4553, uses two biaxial films, it is problematic in that the axes of the films may be often misplaced to cause panel unevenness. In addition, the light leakage in black display is because the triacetyl cellulose film that has heretofore been used as a polarizing plate-protective film put between the liquid-crystal cell and the polarizing film has an in-plane retardation Re of about 5 nm and a thickness-direction retardation Rth of about 50 nm. Accordingly, it is desired to develop a cellulose acylate film having a small in-plane retardation Re and a small thickness-direction retardation Rth and to use it as a protective film for polarizing plate.
Recently, the temperature of liquid-display devices in service is often high owing to the backlight inside them and the devices are often used in a severe condition of high temperature and high humidity or in a severe condition of low humidity, and there occur various problems in that Re and Rth of the protective film of triacetyl cellulose for polarizing plate fluctuate depending on the ambient temperature and humidity and therefore the optically-compensatory capability of the film therefore varies, whereby light may leak away in black display and the images formed may be uneven. The problems are that the aspect ratio of the panel of liquid-crystal display devices differs, that the physical properties of the members constituting the devices naturally differ in the machine direction and in the cross direction thereof, and therefore, in original black display, light may leak away through the around of the frame of the display devices and therefore the image color may change. In addition, the temperature and humidity change causes film curling, and, as a result, it causes other serious problems in that the polarizing plate and even the liquid-crystal display device that comprises the integrated polarizing plate and liquid-crystal cell may warp to thereby cause cell deformation and therefore light leakage and color unevenness owing to the rib interference at the corners of the display panel.
For controlling the curl value, known are methods described in JP-A 2001-131301, JP-A 2001-151902 and JP-A 2001-163994. According to these methods, however, it is still unsatisfactory to control the curl value owing to the ambient humidity change. Therefore, it is desired to develop a film capable of giving a liquid-crystal display device that is free from a problem of environment-dependent geometric characteristic change and optical compensatory function change.